1. Field of the Invwention
This invention relates to dot matrix printers and more particularly to print heads for use in serial dot matrix printers.
2. Description of the Prior Art
Many types of dot matrix printers are now in use as an output device for data processers or similar apparatuses. Among others, serial type dot matrix printers which are capable of imprinting characters and signs of all sorts and suited for high-speed printing operation have been widely used. As is well known, a serial dot matrix printer includes a print head that comprises print hammers each having a thin, elongate print wire fixed to its tip and drive mechanisms therefor and is arranged to imprint characters (or signs) on a piece of paper each in the form of a set of dots by pressing the print wires against an ink ribbon that faces the piece of paper while moving in a predetermined direction relative to the piece of paper. In such a dot matrix printer print head, the print wires, usually seven or nine, are arranged to converge toward each other. A set of hammer drive mechanisms which include permanent magnets and soleniods required for their operation, are used to actuate the print wires, and the drive mechanisms require a considerably large volume of space compared with that required for the set of print hammers. It follows therefore that, in order to increase the speed of operation of such dot matrix printer print head, the hammer drive mechanisms should be made as compact and lightweight as possible. Alternatively, the power supply for the hammer drive mechanisms can be increased.
In Japanese published unexamined patent application No. 51,569/1980 (corresponding U.S. Pat. No. 4,225,250), there is disclosed a serial dot matrix printer print head of a cylindrical sandwich structure adapted for high-speed operation with reduced size and weight and which includes component members, such as a base plate, a ring form of permanent magnet, a print hammer disc and a yoke plate, laid one on another and clamped together as by bolt means, and also a plurality of electromagnetic drivers (each including a core post and an exciting coil mounted thereon) arranged inside the ring magnet circumferentially at regular intervals to actuate the respective print hammers. Such a print head structure, however, is limited in its speed of operation with low power requirements and its productivity cannot be significantly improved to any desired extent. Specifically, with the above-described structure, in order to release any of the print hammers for dot printing operation, which are normally cocked or held attracted on the respective posts under the effect of a flux of a ring magnet, an exciting coil mounted on the associated post must be fed with an electrical pulse signal of a magnitude large enough to produce a neutralizing flux effective to cancel the flux of the magnet flowing in the same flux channel. In addition, since the electromagnetic drive members, each forming a heat source during printing operation, are surrounded by the ring magnet, efficient heat dissipation cannot be expected and a definite limitation is placed on the operation speed of the print head. On the other hand, formation of cooling fins on the ring magnet result in reduction in the productivity of the print head because the machinability of the magnet material is very limited. Further, formation in the ring magnet of a plurality of bolt-receiving apertures, required to clamp together the print head members into a cylindrical sandwich structure, and segmental magnetization of the ring magnet, designed to partly magnetize the core posts and print hammers to maintain the magnitude of the neutralizing field required at a low level, both complicate the process of fabricating such magnet and impair the productivity of the print head structure. Moreover, in the print head structure, the magnet used as a bias means to normally hold the print hammers in their cocked position forms an outer casing wall of the print head and, accordingly, tends to attract any magnetic substances lying thereabouts. These magnetic substances create a danger of the printing operation being impaired.